1. Field of the Invention
The present invention relates to logging-while-drilling ("LWD") tools and to methods of operating logging while drilling. In another aspect, the present invention relates to LWD tools having turbine blades which, when driven by the circulating well fluid provides electrical power to the tool, and to a method of operating such a tool. In even another aspect, the present invention relates to LWD tools having a deflector for deflecting a portion of the circulating well fluid away from the turbine blades, and to a method of operating such a tool.
2. Description of the Related Art
Logging While Drilling Tools (LWD) are used to provide real-time quantitative analysis of sub-surface formations during the actual drilling operation. Typically, these quantitative measurements include: formation resistivity, neutron and density porosity, and acoustic travel time of the formations of interest. Due to the fact that the LWD tool string is an integral part of the bottom hole assembly, it is impractical to connect an umbilical (i.e. wireline) from the surface to provide the electrical power required by the various LWD components.
In the prior art, there have been primarily two sources of electrical power for downhole LWD tools. These include: 1) lithium batteries; and 2) downhole turbine/alternator power supplies. Lithium batteries have been used reliably in both LWD and Measurement While Drilling (MWD) applications for quite some time. The major shortcomings of the lithium batteries are: 1) the batteries have a finite life; 2) they have a limited maximum current rating; 3) once the batteries are "used-up", there are difficulties associated with the proper disposal of the depleted cells; and 4) the batteries tend to be a safety concern if mishandled. Due to the relatively large power requirements of modern LWD tools, turbine/alternator power supplies are commonly used. In turbine/alternator power supplies, mechanical power is extracted from the flow of drilling fluid by means of a fluidic turbine. The rotational output of the turbine is coupled to the input of a permanent magnet alternator which, by means of electronic regulation, is used to power the LWD tool string. Turbine/alternator power supplies have the advantage of providing relatively large amounts of electrical power. This is due to the fact that the flow of drilling fluid provides an extremely large amount of mechanical power available for conversion. Also, turbine/alternator power supplies are able to provide electrical power theoretically for as long as the drilling fluid is circulating, thereby extending the downhole life of the LWD tool string.
There have been numerous shortcomings with turbine/alternator power supplies. Due to the fact that the turbine is extracting mechanical power directly from the drilling fluid flow, a large amount of erosion is typically encountered on and adjacent to the turbine's rotating elements. Depending on the LWD tool size (i.e. outside diameter) a wide range of drilling fluid flow must be accommodated. In order to accommodate the wide flow range typically encountered in LWD tools, several turbine blade arrangements must be adaptable to the turbine/alternator power supply. This obviously adds overall system cost and the possibility of human error in appropriately selecting the turbine blade arrangement required for a given drilling (i.e. flow rate) condition. Also, because the turbine blades are positioned directly in the path of the drilling fluid flow, they are extremely susceptible to jamming or plugging by debris such as pipe scale or "lost circulation materials" commonly encountered in drilling environments.
As an additional shortcoming, turbines of commonly utilized downhole turbine/alternator power supplies are outfitted with blades which occupy the entire flow annulus. These "full-bore" turbines are highly susceptible to plugging or jamming by debris present in the flow. In an effort to reduce the risk of plugging in existing turbines, the blades themselves are designed with large clearances, both radially at the blade tips of the turbine rotor and axially between the turbine stator and rotor, to allow the passage of debris. As a result of these large blade clearances, the turbines themselves are fairly inefficient and extremely susceptible to erosion due to the formation of vorticity.
There is a need in the art for an improved LWD tool/turbine arrangement.
There is another need in the art for a turbine arrangement that is less susceptible to jamming or plugging by debris such as pipe scale or "lost circulation materials" commonly encountered in drilling environments.
There is even another need in the art for an LWD tool turbine arrangement having improved efficiency over prior art LWD tool turbine arrangements.
These and other needs in the art will become apparent to those of skill in the art upon review of this patent specification, including its claims and drawings.